1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device by mounting a semiconductor chip onto a flexible substrate, and also relates to a flexible substrate and a semiconductor device.
2. Description of Related Art
FIG. 1 is a vertical sectional view showing a mount example in a conventional semiconductor device. This semiconductor device is an example applied to a driver of a liquid crystal display. In a flexible substrate 101, a pattern of Cu wiring 103 with a thickness of 12 μm is formed on a base material 102 made of a 40-μm polyimide film. The Cu wiring 103 is plated with about 0.2-μm thick Sn (not shown). A pattern of semiconductor chip connecting electrodes 104 on the flexible substrate 101 is also collectively formed and plated in the same manner as the wiring pattern of Cu wiring 103.
FIG. 2 is an enlarged plan view of a part of the flexible substrate 101 onto which a semiconductor chip 106 (see FIG. 1) is mounted. In order to protect the Cu wiring 103 from pollution and mechanical damage, the flexible substrate 101 is coated with a solder resist 105, whereas the semiconductor chip connecting electrodes 104 are, of course, not coated with the solder resist 105 because they are to be connected to the semiconductor chip 106 (see FIG. 1). It is difficult to open only the portions of the semiconductor chip connecting electrodes 104 by the solder resist film formation using a printing method, and therefore, in general, rectangular portions having lines of the semiconductor chip connecting electrodes 104 as four sides are not coated. The opening in the solder resist 105 is referred to as a device hole 109.
Note that, although not shown in the drawings, an output terminal line for connecting the flexible substrate 101 to a liquid crystal display is also not coated with the solder resist 105.
Au protruding electrodes 107 with a height of 10 μm and an area of 35 μm×80 μm (see FIG. 1), which are provided by plating on the semiconductor chip 106 (see FIG. 1) serving as a liquid crystal driver chip, and the semiconductor chip connecting electrodes 104 as a part of the wiring pattern (103) of the Sn-plated flexible substrate 101 are bonded by Au—Sn eutectic bonding by application of high-temperature heat and pressure. Since the planes of the element surface of the semiconductor chip 106 and the semiconductor chip mount surface of the flexible substrate 101 face each other in this Au—Sn eutectic bond, all the electrodes can be collectively connected under the same condition.
Thus, the semiconductor chip 106 is placed so that the element surface thereof faces the flexible substrate 101, and all the electrodes are connected by Au—Sn eutectic bonding. Conditions of a flip chip connection apparatus for obtaining the Au—Sn eutectic bond are a pressure of 170×10−4 gf/μm2, a tool temperature of 420° C., and a time of 1 second.
After connecting the semiconductor chip 106 (see FIG. 1) to the flexible substrate 101, a gap is generated between the flexible substrate 101 and the semiconductor chip 106 due to the Au protruding electrodes 107 and the semiconductor chip connecting electrodes 104. Therefore, the gap is then filled with synthetic resin 108 so as to protect the Au—Sn eutectic bond, the semiconductor chip 106, etc.
In the process of filling the synthetic resin 108, first, the synthetic resin 108 is continuously dropped from a dispenser onto the flexible substrate 101 along the edge of the semiconductor chip 106. The synthetic resin 108 spread on the flexible substrate 101 comes into contact with the edge of the semiconductor chip 106, and then enters and fills the interface between the flexible substrate 101 and the semiconductor chip 106 by capillary action. Thereafter, mounting of the semiconductor chip 106 onto the flexible substrate 101 is completed by thermosetting the synthetic resin 108.
The above-mentioned mounting configuration is referred to as COF (Chip On Film). This COF and TCP (Tape Carrier Package), which also uses a flexible substrate, are suitable for the mounting of semiconductor chips in apparatuses which are required to be light, thin and small, such as the mounting of a liquid crystal driver chip.
A similar structure is also described in a conventional example disclosed in Japanese Patent Application Laid-Open No. 2001-176918. As described in this application, in general, the connection section between a liquid crystal panel and electronic parts such as a semiconductor chip is not coated with a solder resist.
A liquid crystal driver chip is an IC chip for driving a liquid crystal display. The liquid crystal display includes liquid crystal filled in the space between two transparent plates. Since the liquid crystal has the problem of deterioration generated when the same potential is continuously applied, there have been developed liquid crystal driving methods in which the same potential is not continuously applied to the liquid crystal itself even when a still image is displayed on the screen, for example. Such driving methods include a line inversion method and a dot inversion method, and the dot inversion method is mainly used at present since it has the advantage to obtain a clearer screen image.
In the dot inversion method, a potential of different polarity is applied to each of adjacent wires of source wiring connected to a TFT (Thin Film Transistor), and the potential is inverted according to a clock cycle so as not to continuously apply a constant voltage to the liquid crystal. In this method, a voltage of a maximum of ten and several volts is sometimes applied between adjacent wires.
This voltage is supplied as an analog signal from the liquid crystal driver chip. Of course, a flexible substrate on which the liquid crystal driver chip is mounted receives this voltage.
When the flexible substrate is left under a high humidity environment in the state in which a voltage of ten and several volts is applied between adjacent wires on the flexible substrate, the interface resin absorbs moisture, and the moisture eventually reaches the flexible substrate.
Consequently, moisture is present between the wires to which the voltage is applied on the flexible substrate, and an ion migration phenomenon in which metal atoms migrate with a flow of current occurs, and causes the problem that a short circuit finally occurs between the wires to which the voltage of ten and several volts is applied. In particular, this phenomenon is noticeable in a portion that is not coated with the solder resist because the portion comes into contact directly with the moisture which has penetrated.
When the dot inversion method is employed as the liquid crystal driving method for the above-mentioned conventional flexible substrate, the reliability depends on the moisture absorbing rate of the interface resin, and it is actually impossible to prevent ion migration between wires in a portion that is not coated with the solder resist. Although there is no portion where the Cu wiring is exposed, it is difficult to prevent ion migration by only the synthetic resin since moisture penetrates into the synthetic resin and reaches the flexible substrate.